Are flashing headlights as a cue for drug deals/murder/etc? Why were two guys circling my car repetitively?

If you got the plate of the car, report it to police. This could be just some punks. This could be a tragedy waiting to happen. If you are concerned enough to ask a bunch of strangers, tell the police and have your father talk to them too. These guys may be responsible for other things you are not aware of. Better safe than sorry; tell the police.